Method for driving liquid crystal display device

ABSTRACT

A display device including a light-transmitting pixel portion and a reflective pixel portion is provided. In a moving image display period, the light-transmitting pixel portion performs an image display and a reflective pixel portion performs a black display. In a still image display period, the reflective pixel portion performs a display in response to black-and-white grayscale image signals.

TECHNICAL FIELD

The present invention relates to a method for driving a liquid crystaldisplay device. Alternatively, the present invention relates to a liquidcrystal display device. Further alternatively, the present inventionrelates to an electronic device including a liquid crystal displaydevice.

BACKGROUND ART

Liquid crystal display devices are widely used in large display devicessuch as television receivers and small display devices such as mobilephones. Products with higher added values are required and are beingdeveloped. In recent years, in view of increase in concern about globalenvironment and improvement in convenience of mobile equipment,development of liquid crystal display devices with low power consumptionhas attracted attention.

Non-Patent Document 1 discloses a structure of a liquid crystal displaydevice in which refresh rates differ between the case of moving imagedisplay and the case of still image display for reduction in powerconsumption of the liquid crystal display device.

Non-Patent Document 2 discloses a structure of a semi-transmissiveliquid crystal display device in which color image display performed byfield sequential driving and monochrome image display performed byturning off a backlight and using reflected light are switched forreduction in power consumption of the liquid crystal display device.

REFERENCE

-   [Non-Patent Document 1] Kazuhiko Tsuda et al., IDW'02, pp. 295-298-   [Non-Patent Document 2] Ying-hui Chen et al., IDW'09, pp. 1703-1707

DISCLOSURE OF INVENTION

According to Non-Patent Document 1, power consumption can be reduced bylowering the refresh rate in displaying a still image. However, sincepower consumption of a liquid crystal display device largely depends onlighting of a backlight, the structure of Non-Patent Document 1 has aproblem in that power consumption is not sufficiently reduced. Thestructure of Non-Patent Document 2 has a problem in that fieldsequential driving in a semi-transmissive liquid crystal display deviceleads to insufficient contrast of a display image due to lightscattering or the like in a reflective pixel portion, especially underintense outside light.

Thus, an object of an embodiment of the present invention is to suppressreduction in contrast due to light scattering or the like in areflective pixel portion and to reduce power consumption.

An embodiment of the present invention is a method for driving asemi-transmissive liquid crystal display device including a plurality ofpixels. Each of the pixels has a light-transmitting pixel portion and areflective pixel portion. The light-transmitting pixel portion includesa first pixel transistor whose first terminal is electrically connectedto a signal line and whose gate is electrically connected to a scanline, and a first liquid crystal element and a first capacitor which areelectrically connected to a second terminal of the first pixeltransistor. The reflective pixel portion includes a second pixeltransistor whose first terminal is electrically connected to the secondterminal of the first pixel transistor and whose gate is electricallyconnected to a selection line, and a second liquid crystal element and asecond capacitor which are electrically connected to a second terminalof the second pixel transistor. In the method for driving thesemi-transmissive liquid crystal display device, in a first period, thefirst pixel transistor is turned on, the second pixel transistor isturned off, and a first image signal is supplied to the first liquidcrystal element and the first capacitor from the signal line. In asecond period, display is performed in the light-transmitting pixelportion in response to the first image signal supplied in the firstperiod. In a third period, the first pixel transistor is turned on, thesecond pixel transistor is turned on, and a signal for black display issupplied to the second liquid crystal element and the second capacitorfrom the signal line in the reflective pixel portion. The first to thirdperiods are repeated so that a moving image is displayed. In a fourthperiod, the first pixel transistor is turned on, the second pixeltransistor is turned on, and a second image signal is supplied to thefirst liquid crystal element, the first capacitor, the second liquidcrystal element, and the second capacitor from the signal line. In afifth period, display is performed in the reflective pixel portion inresponse to the second image signal supplied in the fourth period. Thefourth period and the fifth period are repeated so that a still image isdisplayed.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which the first image signal suppliedin the first period is an image signal corresponding to any color of R,G, and B, and backlights which emit respective colors of R, G, and B aresequentially operated in the second period.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which the signal for black displaysupplied in the third period is supplied to each pixel by linesequential driving.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which the second image signal is animage signal for displaying an image at a lower grayscale level than animage of the first image signal.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which time for displaying one image inthe fourth and fifth periods is longer than time for displaying oneimage in the first to third periods.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which supply of a driver circuitcontrol signal for driving the scan line and the signal line is stoppedin the fifth period.

According to an embodiment of the present invention, reduction incontrast due to light scattering or the like in a reflective pixelportion can be suppressed and power consumption can be reduced withoutmaking the structure complicated, for example, increase in the number ofdriver circuits, wirings, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention;

FIG. 2 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention;

FIG. 3 is a chart showing operation of a liquid crystal display deviceof an embodiment of the present invention;

FIGS. 4A and 4B are charts showing operation of a liquid crystal displaydevice of an embodiment of the present invention;

FIGS. 5A to 5E are diagrams illustrating a liquid crystal display deviceof an embodiment of the present invention;

FIGS. 6A and 6B are diagrams illustrating a liquid crystal displaydevice of an embodiment of the present invention;

FIG. 7 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention; and

FIGS. 8A and 8B are diagrams illustrating an electronic device of anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention can beimplemented in many different modes, and it is easily understood bythose skilled in the art that modes and details of the present inventioncan be modified in various ways without departing from the spirit andthe scope of the present invention. Therefore, the present invention isnot construed as being limited to the description of the embodiments.Note that in structures of the present invention described below,reference numerals denoting the same portions are used in common indifferent drawings.

Note that the size, the thickness of a layer, the waveform of a signal,and a region of components illustrated in the drawings and the like inthe embodiments are exaggerated for simplicity in some cases. Therefore,the embodiments of the present invention are not limited to such scales.

Note that in this specification, terms such as “first”, “second”,“third”, and “N-th” (N is a natural number) are used in order to avoidconfusion among components and do not limit the components numerically.

Embodiment 1

In this embodiment, a method for driving a liquid crystal display devicewill be described with reference to circuit diagrams of a pixel of theliquid crystal display device, timing charts showing operation thereof,and the like.

First, a configuration will be described with reference to FIG. 1 whichis a circuit diagram of a pixel. FIG. 1 illustrates a pixel 100, a scanline 101 (also referred to as a gate line), a signal line 102 (alsoreferred to as a data line), and a selection line 103. The pixel 100 hasa light-transmitting pixel portion 104 and a reflective pixel portion105. The light-transmitting pixel portion 104 includes a first pixeltransistor 106, a first liquid crystal element 107, and a firstcapacitor 108. The reflective pixel portion 105 includes a second pixeltransistor 109, a second liquid crystal element 110, and a secondcapacitor 111.

In the light-transmitting pixel portion 104, a first terminal of thefirst pixel transistor 106 is connected to the signal line 102 and agate of the first pixel transistor 106 is connected to the scan line101. A first electrode (pixel electrode) of the first liquid crystalelement 107 is connected to a second terminal of the first pixeltransistor 106, and a second electrode (counter electrode) of the firstliquid crystal element 107 is connected to a common potential line 112(common line). A first electrode of the first capacitor 108 is connectedto the second terminal of the first pixel transistor 106, and a secondelectrode of the first capacitor 108 is connected to a capacitor line113.

In the reflective pixel portion 105, a first terminal of the secondpixel transistor 109 is connected to the second terminal of the firstpixel transistor 106 and a gate of the second pixel transistor 109 isconnected to the selection line 103. A first electrode (pixel electrode)of the second liquid crystal element 110 is connected to a secondterminal of the second pixel transistor 109, and a second electrode(counter electrode) of the second liquid crystal element 110 isconnected to the common potential line 112. A first electrode of thesecond capacitor 111 is connected to the second terminal of the secondpixel transistor 109, and a second electrode of the second capacitor 111is connected to the capacitor line 113.

Note that each of the first pixel transistor 106 and the second pixeltransistor 109 is preferably a transistor including an oxidesemiconductor layer. The oxide semiconductor is made to be intrinsic(i-type) by removal of hydrogen that is an n-type impurity to bepurified so that impurities that are not main components of the oxidesemiconductor are included as little as possible. Note that a purifiedoxide semiconductor includes extremely few carriers (close to zero), andthe carrier concentration thereof is lower than 1×10¹⁴/cm³, preferablylower than 1×10¹²/cm³, more preferably 1×10¹¹/cm³. Since the oxidesemiconductor includes extremely few carriers, the off-state current ofthe transistor can be reduced. Specifically, in a transistor includingthe above oxide semiconductor layer, the off-state current permicrometer in channel width at room temperature can be reduced to lessthan or equal to 10 aA/μm (1×10⁻¹⁷ A/μm), further to less than or equalto 1 aA/μm (1×10⁻¹⁸ A/μm), still further to less than or equal to 10zA/μm (1×10⁻²⁰ A/μm). That is to say, in circuit design, the oxidesemiconductor can be regarded as an insulator when the transistor isoff. In the pixel 100 that is a pixel including the transistors whichare formed using the oxide semiconductor and whose off-state current isextremely small, an image can be maintained even when the number oftimes of writing of an image signal (also referred to as video voltage,a video signal, or video data) is small and thus the refresh rate can belowered. Therefore, a period in which a driver circuit for driving thescan line and the signal line is stopped can be provided and powerconsumption can be reduced.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor includes a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain may change depending on the structure, theoperating condition, and the like of the transistor, it is difficult todefine which is a source or a drain. Therefore, in this document (thespecification, the claims, the drawings, and the like), a regionfunctioning as a source and a drain is not called the source or thedrain in some cases. In such a case, for example, one of the source andthe drain may be referred to as a first terminal and the other thereofmay be referred to as a second terminal. Alternatively, one of thesource and the drain may be referred to as a first electrode and theother thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

Note that when it is explicitly described that “A and B are connected,”the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

Note that voltage refers to a potential difference between a givenpotential and a reference potential (e.g., a ground potential) in manycases. Accordingly, voltage, a potential, and a potential difference canbe referred to as a potential, voltage, and a voltage difference,respectively.

Note that a common potential supplied to the common potential line 112may be any potential as long as it serves as a reference with respect toa potential of an image signal supplied to the first electrode of theliquid crystal element, and may be a ground potential, for example.

Note that an image signal may be appropriately inverted in accordancewith dot inversion driving, source line inversion driving, gate lineinversion driving, frame inversion driving, or the like to be input toeach pixel. Note also that the image signal is referred to by anothername such as a first image signal or a second image signal in somecases, depending on the kind of an image to be displayed.

Note that the potential of the capacitor line 113 may be the same as thecommon potential. Alternatively, another signal may be supplied to thecapacitor line 113.

Note that the second electrodes of the first liquid crystal element 107and the second liquid crystal element 110 are preferably provided tooverlap with the first electrodes of the first liquid crystal element107 and the second liquid crystal element 110. The first electrodes andthe second electrodes of the liquid crystal elements may each have ashape including a variety of opening patterns. As a liquid crystalmaterial provided between the first electrodes and the second electrodesin the liquid crystal elements, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal, ferroelectric liquid crystal,anti-ferroelectric liquid crystal, or the like may be used. These liquidcrystal materials exhibit a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions. Alternatively, liquid crystal exhibiting a blue phase forwhich an alignment film is unnecessary may be used.

Note that the first electrode of the first liquid crystal element 107 inthe light-transmitting pixel portion 104 is formed using alight-transmitting material. As examples of the light-transmittingmaterial, indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide(IZO), zinc oxide to which gallium is added (GZO), and the like aregiven. On the other hand, a metal electrode with high reflectivity isused as the first electrode of the second liquid crystal element 110 inthe reflective pixel portion 105. Specifically, aluminum, silver, or thelike is used. In addition, outside light can be reflected irregularly bymaking a surface of the pixel electrode of the second liquid crystalelement 110 uneven. Note that the first electrode, the second electrode,and the liquid crystal material are collectively referred to as a liquidcrystal element in some cases.

Next, FIG. 2 is a schematic view of a liquid crystal display devicewhich includes the pixel 100 having the configuration illustrated inFIG. 1. In the configuration illustrated in FIG. 2, a pixel portion 151,a scan line driver circuit 152 (also referred to as a gate line drivercircuit), a signal line driver circuit 153 (also referred to as a dataline driver circuit), and a terminal portion 154 are provided over asubstrate 150.

Note that in FIG. 2, the scan line 101 is driven so that on/off of thefirst pixel transistor 106 is controlled by the scan line driver circuit152. An image signal to be supplied to the first liquid crystal element107 or the second liquid crystal element 110 is supplied to the signalline 102 from the signal line driver circuit 153. A selection signalthat controls on/off of the second pixel transistor 109 is supplied tothe selection line 103 from the terminal portion 154. Note that the scanline 101 and the selection line 103 are perpendicular to each other inFIG. 2, but may be arranged to be in parallel to each other.

The scan line driver circuit 152 and the signal line driver circuit 153are preferably provided over the substrate over which the pixel portion151 is formed; however, these are not necessarily formed over thesubstrate over which the pixel portion 151 is formed. When the scan linedriver circuit 152 and the signal line driver circuit 153 are providedover the substrate over which the pixel portion 151 is formed, thenumber of the connection terminals for connection to the outside and thesize of the liquid crystal display device can be reduced.

Note that the plurality of pixels 100 is provided (arranged) in a matrixform in the pixel portion 151. Here, description that pixels areprovided (arranged) in a matrix form includes the case where the pixelsare provided in a straight line and the case where the pixels areprovided in a jagged line, in a longitudinal direction or a lateraldirection.

In addition to the selection signal supplied to the selection line 103,signals for controlling the scan line driver circuit 152 and the signalline driver circuit 153 (a high power supply potential V_(dd), a lowpower supply potential V_(ss), a start pulse SP, and a clock signal CK,which are hereinafter referred to as driver circuit control signals),fixed potentials supplied to the common potential line 112 and thecapacitor line 113, and the like are supplied from the terminal portion154. Note that the scan line driver circuit 152 and the signal linedriver circuit 153 to which the driver circuit control signals aresupplied may each include a shift register circuit in which flip-flopcircuits or the like are cascaded. As for the selection signal suppliedto the selection line 103, the same signal may be supplied all at onceto the selection line 103 connected to each pixel, which is differentfrom the case of a scan line or a signal line through which signals aresequentially supplied to a plurality of wirings.

Next, operation of the liquid crystal display device will be describedwith reference to FIG. 3, FIGS. 4A and 4B, and FIGS. 5A to 5E, inaddition to FIG. 2.

As shown in FIG. 3, the operation of the liquid crystal display deviceis roughly divided into a moving image display period 301 and a stillimage display period 302. Note that the moving image display period 301and the still image display period 302 may be switched by supplying asignal for switching the periods from the outside or by judging themoving image display period 301 or the still image display period 302 onthe basis of an image signal.

The cycle of one frame period (or frame frequency) is preferably lessthan or equal to 1/60 sec (higher than or equal to 60 Hz) in the movingimage display period 301. The frame frequency is increased, so thatflickering is not sensed by a viewer of an image. In the still imagedisplay period 302, the cycle of one frame period is extremely long, forexample, longer than or equal to one minute (lower than or equal to0.017 Hz), so that eyestrain can be alleviated as compared to the casewhere the same image is switched plural times.

When an oxide semiconductor is used for semiconductor layers of thefirst pixel transistor 106 and the second pixel transistor 109, carriersin the oxide semiconductor can be extremely few as described above andthus the off-state current can be reduced. Accordingly, an electricalsignal such as the image signal can be held for a longer time in thepixel, and a writing interval can be set longer. Therefore, the cycle ofone frame period can be set longer, and the frequency of refreshoperation in the still image display period 302 can be reduced, wherebyan effect of suppressing power consumption can be further increased.

In the moving image display period 301 in FIG. 3, a color moving imagecan be displayed by field sequential driving, for example. Note thatcolor display may be performed using a color filter. In order to displaya moving image by field sequential driving, the driver circuit controlsignals are supplied to the scan line driver circuit 152 and the signalline driver circuit 153. In the moving image display period 301 in FIG.3, a backlight used for the color display by field sequential driving isoperated. Thus, a color moving image can be displayed on a displaypanel.

In the moving image display period 301, an image signal is supplied fromthe signal line driver circuit 153 so that color display (denoted byCOLOR in the drawing) is performed in the light-transmitting pixelportion 104, and an image signal is supplied from the terminal portion154 so that display at a black grayscale level (denoted by BK in thedrawing) is performed in the reflective pixel portion 105. Thus, thecontrast in the light-transmitting pixel portion 104, which is reducedby light scattering caused by irradiation of outside light on thereflective pixel portion 105, can be recovered.

In the still image display period 302 in FIG. 3, an image signal issupplied so that a black-and-white grayscale (denoted by BK/W in thedrawing) is displayed depending on whether reflected light istransmitted or not, whereby a still image can be displayed. In the stillimage display period 302, the driver circuit control signals aresupplied only when the black-and-white grayscale image signals arewritten, and supply of the driver circuit control signals is partly orcompletely stopped in a period in which the image signal which has beenwritten is held, that is, in a period except the period in which theblack-and-white grayscale image signal is written. Therefore, powerconsumption can be reduced in the still image display period 302 owingto the period in which the supply of the driver circuit control signalsis stopped. Moreover, display comes to be visible by utilizing reflectedlight of outside light in the still image display period 302 in FIG. 3;therefore, the backlight is not operated. Thus, a black-and-whitegrayscale still image can be displayed on the display panel.

As for the stop of the supply of the driver circuit control signals, inthe case where the holding period of the image signal which has beenwritten is short, a configuration in which supply of the high powersupply potential V_(dd) and the low power supply potential V_(ss) is notstopped may be originally employed. This is because increase in powerconsumption due to repetition of stop and start of supply of the highpower supply potential V_(dd) and the low power supply potential V_(ss)can be reduced, which is favorable.

Next, the moving image display period 301 and the still image displayperiod 302 of FIG. 3 will be described in detail with reference totiming charts of FIGS. 4A and 4B, respectively. The timing charts ofFIGS. 4A and 4B are exaggerated for description.

First, FIG. 4A will be described. FIG. 4A shows the driver circuitcontrol signals supplied to the scan line driver circuit 152 and thesignal line driver circuit 153, image signals, and the state of thebacklights in one frame period of the moving image display period 301,as an example. As for the backlights, the case where lights emittingthree colors of red (R), green (G), and blue (B) are sequentially turnedon is described. By using LEDs as the backlights, lower powerconsumption and longer lifetime can be achieved.

In the moving image display period 301, a moving image is displayed byfield sequential driving; therefore, operation in the light-transmittingpixel portion 104 is performed in such a manner that an image signal forred (R) display is written into each pixel first, a backlight of R isthen turned on, an image signal for green (G) display is written intoeach pixel next, a backlight of G is then turned on, an image signal forblue (B) display is written into each pixel next, and a backlight of Bis then turned on. Next, in the moving image display period 301, afterthe image signals of R, G, and B are written and the backlights of R, G,and B are turned on, operation is performed so that an image signal fordisplay at a black grayscale level is supplied to the reflective pixelportion 105. Further, in the moving image display period 301, the drivercircuit control signals are supplied to driver circuits, so that boththe scan line driver circuit 152 and the signal line driver circuit 153are operated.

In short, the moving image display period 301 can be roughly dividedinto an image signal writing period (T1 in FIG. 4A, which is alsoreferred to as a first period), a backlight lighting period (T2 in FIG.4A, which is also referred to as a second period), and a black grayscalesignal writing period (T3 in FIG. 4A, which is also referred to as athird period).

By repeating the above operation so that image signals are changed, aviewer can perceive color display of a moving image. Note that the orderof R, G, and B in FIG. 4A may be a different order or display may beperformed using more colors. The black grayscale signal writing period,which is the third period T3, is provided once in one frame period inFIG. 4A, but may be provided once in a plurality of frame periods.

Note that the supply of the image signal for display at a blackgrayscale level to the reflective pixel portion 105 may be performedbefore the image signal writing period and the backlight lightingperiod. Thus, the contrast in the light-transmitting pixel portion 104,which is reduced by light scattering caused by irradiation of outsidelight on the reflective pixel portion 105, can be recovered.

Next, FIG. 4B will be described. Similarly to FIG. 4A, FIG. 4B shows thedriver circuit control signals supplied to the scan line driver circuit152 and the signal line driver circuit 153, an image signal, and thestate of the backlights in one frame period of the still image displayperiod 302.

In the still image display period 302, an image signal for displaying ablack-and-white grayscale image depending on whether reflected light istransmitted or not is supplied. At this time, the backlights are notoperated, and the driver circuit control signals are supplied to drivercircuits, so that both the scan line driver circuit 152 and the signalline driver circuit 153 are operated. Next, the image signal fordisplaying a black-and-white grayscale image which has been written isheld, so that a still image is displayed. At this time, an additionalimage signal is not written, the backlights are not operated, and thedriver circuit control signals are not supplied. Therefore, powerconsumed by the backlights and the driver circuit control signals can bereduced; thus, lower power consumption can be achieved. As for theholding of the still image, the image signal written into a pixel isheld by a pixel transistor whose off-state current is extremely small;therefore, the black-and-white grayscale still image can be held forlonger than or equal to one minute. In addition, the still image may beheld in the following manner: before the level of the image signal heldis lowered after a certain period of time, a new still image signalwhich is the same image signal as the still image signal of the previousperiod is written (refresh operation) and the still image is held again.

The still image display period 302 can be roughly divided into a stillimage signal writing period (T4 in FIG. 4B, which is also referred to asa fourth period) and a still image signal holding period (T5 in FIG. 4B,which is also referred to as a fifth period).

Next, how the pixel 100 in FIG. 1 is operated in the periods T1 to T5 inFIGS. 4A and 4B will be described with reference to FIGS. 5A to 5E whichillustrate signals and on/off of pixel transistors. Although not allcomponents are denoted by reference numerals in FIGS. 5A to 5E,description is given using the same reference numerals as FIG. 1.Further, dotted arrows in FIGS. 5A to 5E are shown to facilitateunderstanding of signal flow. “ON” and “OFF” in FIGS. 5A to 5E representon and off of the pixel transistors, respectively. “COLOR” in FIGS. 5Ato 5E is shown to facilitate understanding of the state where a colorimage signal (a first image signal) is supplied to or held in the signalline, the light-transmitting pixel portion, or the reflective pixelportion. Similarly, “BK” represents a black grayscale image signal (animage signal for black display), and “BK/W” represents a black-and-whiteimage signal (a second image signal).

Note that the black-and-white image signal, which is the second imagesignal, refers to a grayscale signal that is a signal of a grayscaleimage or a monochrome image. By using an image signal at a lowergrayscale level than the first image signal as the second image signal,change in the grayscale in the still image signal holding period can beless likely to be perceived.

First, in the period T1 illustrated in FIG. 5A, that is, in the imagesignal writing period, the scan line 101 is controlled so that the firstpixel transistor 106 is turned on, the first image signal (denoted byCOLOR in the drawing) is supplied to the signal line 102, and the firstimage signal is written into the first liquid crystal element 107 in thelight-transmitting pixel portion 104; thus, the alignment of liquidcrystal in the light-transmitting pixel portion 104 is controlled. Atthis time, in the reflective pixel portion 105, the selection line 103is controlled so that the second pixel transistor 109 is turned off, thefirst image signal of the signal line 102 is not written into the secondliquid crystal element 110, and a black grayscale image signal (denotedby BK in the drawing) which has been written in the previous frameperiod is held; thus, the second liquid crystal element 110 in thereflective pixel portion 105 is controlled.

Then, in the period T2 illustrated in FIG. 5B, that is, in the backlightlighting period, the scan line 101 is controlled so that the first pixeltransistor 106 is turned off, and light from the backlight istransmitted or not transmitted depending on the alignment of liquidcrystal corresponding to the first image signal (denoted by COLOR in thedrawing) which has been written in the image signal writing period inthe light-transmitting pixel portion 104. At this time, in thereflective pixel portion 105, the selection line 103 is controlled sothat the second pixel transistor 109 is turned off, and the blackgrayscale image signal (denoted by BK in the drawing) which has beenwritten in the previous frame period is held; thus, the second liquidcrystal element 110 in the reflective pixel portion 105 is controlled.

Next, in the period T3 illustrated in FIG. 5C, that is, in the blackgrayscale signal writing period, the scan line 101 is controlled so thatthe first pixel transistor 106 is turned on, a signal for black display(denoted by BK in the drawing) is supplied to the signal line 102, andthe signal for black display is written into the first liquid crystalelement 107 in the light-transmitting pixel portion 104; thus, thealignment of liquid crystal in the light-transmitting pixel portion 104is controlled. At this time, in the reflective pixel portion 105, theselection line 103 is controlled so that the second pixel transistor 109is turned on, and the black grayscale image signal of the signal line102 is written into the second liquid crystal element 110; thus, thesecond liquid crystal element 110 in the reflective pixel portion 105 iscontrolled. Note that after the black grayscale image signal is writteninto the first liquid crystal element 107 and the second liquid crystalelement 110, both the first pixel transistor 106 and the second pixeltransistor 109 may be turned off.

Note that the writing of the signal for black display into the firstliquid crystal element 107 and the second liquid crystal element 110 inthe period T3 may be performed all at once in all the pixels, or may beperformed row by row (line sequential driving). By writing the signalfor black display all at once or by writing the signal for black displayby line sequential driving, the period T3 can be shortened and the imagequality can be improved.

Next, in the period T4 illustrated in FIG. 5D, that is, in the stillimage signal writing period, the scan line 101 is controlled so that thefirst pixel transistor 106 is turned on, the second image signal(denoted by BK/W in the drawing) is supplied to the signal line 102, andthe signal for black-and-white display is written into the first liquidcrystal element 107 in the light-transmitting pixel portion 104; thus,the alignment of liquid crystal in the light-transmitting pixel portion104 is controlled. At this time, in the reflective pixel portion 105,the selection line 103 is controlled so that the second pixel transistor109 is turned on, and the second image signal of the signal line 102 iswritten into the second liquid crystal element 110; thus, the secondliquid crystal element 110 in the reflective pixel portion 105 iscontrolled.

Then, in the period T5 illustrated in FIG. 5E, that is, in the stillimage signal holding period, the scan line 101 is controlled so that thefirst pixel transistor 106 is turned off and the alignment of liquidcrystal is controlled in response to the second image signal (denoted byBK/W in the drawing) which has been written in the still image signalwriting period in the light-transmitting pixel portion 104. At thistime, in the reflective pixel portion 105, the selection line 103 iscontrolled so that the second pixel transistor 109 is turned off, andthe second image signal (denoted by BK/W in the drawing) which has beenwritten in the still image signal writing period is held; thus, thesecond liquid crystal element 110 in the reflective pixel portion 105 iscontrolled.

Note that in the still image signal writing period illustrated in FIG.5D, the second image signal is written into the reflective pixel portion105 and the second image signal is also written into thelight-transmitting pixel portion 104. Although the backlight is notoperated in the still image signal holding period illustrated in FIG.5E, an image might be dark and difficult to see owing to insufficientreflection of light in the reflective pixel portion 105, depending onthe environment or the like. In such a case, the visibility can besecured by operating the backlight and switching display of thereflective pixel portion 105 to the display of the light-transmittingpixel portion 104 into which the second image signal at the samegrayscale level has been written. The switching of an operating stateand a non-operating state of the backlight may be performed only whenthe visibility is insufficient; therefore, an optical sensor or the likemay be additionally provided and the switching may be performed inaccordance with the illuminance of the environment. Note that theoperating state and the non-operating state of the backlight may beswitched by manual operation with a switch or the like. Further, byusing an oxide semiconductor for the first pixel transistor 106 and thesecond pixel transistor 109, the off-state current thereof can bereduced. Reduction in off-state current leads to a long still imagesignal holding period; therefore, the use of an oxide semiconductor ispreferable for reduction in power consumption.

In the still image signal holding period, the frequency of operationsuch as writing of an image signal can be reduced. When seeing an imageformed by writing image signals a plurality of times, the human eyesrecognize images switched a plurality of times, which might lead toeyestrain. With a structure in which the frequency of writing of imagesignals is reduced as described in this embodiment, eyestrain can bealleviated.

In the above-described manner, according to an embodiment of the presentinvention, reduction in contrast due to light scattering or the like ina reflective pixel portion can be suppressed and power consumption canbe reduced without making the structure complicated, for example,increase in the number of driver circuits, wirings, and the like.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, structures of a top view and a cross-sectional viewcorresponding to the circuit diagram of the pixel of the liquid crystaldisplay device illustrated in FIG. 1 of Embodiment 1 will be described.

FIGS. 6A and 6B are a top view and a cross-sectional view, respectively,in the case where inverted staggered transistors are used as the firstpixel transistor 106 and the second pixel transistor 109 described inEmbodiment 1. The cross-sectional view of a pixel transistor illustratedin FIG. 6B corresponds to a cross section along line A-A′ in the topview of the pixel illustrated in FIG. 6A.

First, an example of a layout of a pixel of a liquid crystal displaydevice will be described with reference to FIG. 6A. Note that FIGS. 6Aand 6B illustrate a structure applied to the pixel 100 in FIG. 1described in Embodiment 1.

The pixel in FIG. 6A that can be applied to the liquid crystal displaydevice of Embodiment 1 includes a scan line 801, a signal line 802, aselection line 803, a capacitor line 804, a first pixel transistor 805,a first pixel electrode 806, a first capacitor 807, a second pixeltransistor 808, a second pixel electrode 809, and a second capacitor 810as components corresponding to those in FIG. 1. The components areformed using a conductive layer 851, a semiconductor layer 852, aconductive layer 853, a transparent conductive layer 854, a reflectiveconductive layer 855, a contact hole 856, and a contact hole 857.

The conductive layer 851 has a region that functions as a gate electrodeor a scan line. The semiconductor layer 852 has regions that function assemiconductor layers of the pixel transistors. The conductive layer 853has regions that function as a wiring and sources and drains of thepixel transistors. The transparent conductive layer 854 has a regionthat functions as a pixel electrode of a first liquid crystal element.The reflective conductive layer 855 has a region that functions as apixel electrode of a second liquid crystal element. The conductive layer851 and the conductive layer 853 are connected to each other through thecontact hole 856. The conductive layer 853 and the transparentconductive layer 854 or the conductive layer 853 and the reflectiveconductive layer 855 are connected to each other through the contacthole 857.

Note that FIG. 7 illustrates a layout of a pixel in which the reflectiveconductive layer 855 is not shown. As illustrated in FIG. 7, the secondpixel transistor 808 and the second capacitor 810 are provided tooverlap with the reflective conductive layer 855. The second capacitor810 is provided in a position overlapping with the reflective conductivelayer 855; thus, the capacitance can be increased without reduction inthe aperture ratio.

In order to reflect incident outside light irregularly, the reflectiveconductive layer 855 is preferably subjected to treatment for making asurface thereof uneven.

In the layouts of the pixels in FIG. 6A and FIG. 7, the first pixelelectrode 806 and the signal line 802 are provided to be apart from eachother. By providing the first pixel electrode 806 and the signal line802 to be apart from each other, variation in the potential of the firstpixel electrode 806 due to variation in the potential of the signal linecan be reduced.

In the layouts of the pixels in FIG. 6A and FIG. 7, the conductive layer851 is preferably provided so as to surround the first pixel electrode806. With the structure in which the first pixel electrode 806 issurrounded by the conductive layer 851, a light-blocking portion (blackmatrix) which is provided so as to surround the first pixel electrode806 can be omitted. Moreover, it is preferable that the conductive layer851 be provided between the transparent conductive layer 854 and thereflective conductive layer 855 because a difference in height betweensurfaces of the transparent conductive layer 854 and the reflectiveconductive layer 855 can be reduced.

In the layouts of the pixels in FIG. 6A and FIG. 7, the selection line803 and the capacitor line 804 are provided in parallel to the signalline 802. By providing the selection line 803, the capacitor line 804,and the signal line 802 in parallel to one another, the capacitance inan intersection between the wirings can be reduced. Accordingly, noise,delay of a signal, distortion of a signal waveform, or the like can bereduced.

Next, the structure of the cross-sectional view of FIG. 6B will bedescribed. In this embodiment, a method for forming a transistorparticularly when a semiconductor layer is formed using an oxidesemiconductor will be described. FIG. 6B illustrates a transistorincluding an oxide semiconductor as a semiconductor layer. An advantageof using an oxide semiconductor is that high mobility and low off-statecurrent can be obtained in relatively easy and low-temperatureprocesses; needless to say, another semiconductor may be used.

A transistor 410 illustrated in FIG. 6B is a kind of bottom-gatetransistors, and is also referred to as an inverted staggeredtransistor. Note that there is no particular limitation on a structureof a transistor which can be applied to a liquid crystal display devicedisclosed in this specification. For example, a top-gate staggeredstructure, a bottom-gate staggered structure, a top-gate planarstructure, a bottom-gate planar structure, or the like can be used. Thetransistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned above and below a channel region with gate insulating layersprovided therebetween.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. In addition, an insulating layer 407 whichcovers the transistor 410 and is stacked over the oxide semiconductorlayer 403 is provided. A protective insulating layer 409 is formed overthe insulating layer 407.

In this embodiment, as described above, the oxide semiconductor layer403 is used as a semiconductor layer. As an oxide semiconductor used forthe oxide semiconductor layer 403, an In—Sn—Ga—Zn—O-based metal oxidewhich is a four-component metal oxide; an In—Ga—Zn—O-based metal oxide,an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, aSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, or aSn—Al—Zn—O-based metal oxide which is a three-component metal oxide; anIn—Zn—O-based metal oxide, a Sn—Zn—O-based metal oxide, an Al—Zn—O-basedmetal oxide, a Zn—Mg—O-based metal oxide, a Sn—Mg—O-based metal oxide,or an In—Mg—O-based metal oxide which is a two-component metal oxide; anIn—O-based metal oxide, a Sn—O-based metal oxide, a Zn—O-based metaloxide, or the like can be used. Further, SiO₂ may be included in asemiconductor of the above metal oxide. Here, for example, anIn—Ga—Zn—O-based metal oxide is an oxide including at least In, Ga, andZn, and there is no particular limitation on the composition ratiothereof. Further, the In—Ga—Zn—O-based metal oxide may include anelement other than In, Ga, and Zn.

For the oxide semiconductor layer 403, a thin film represented by thechemical formula, InMO₃(ZnO)_(m) (m>0) can be used. Here, M representsone or more metal elements selected from Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the transistor 410 including the oxide semiconductor layer 403, acurrent value in an off state (off-state current value) can be reduced.Therefore, an electrical signal of image data or the like can be heldfor a longer time, so that a writing interval can be set longer.Accordingly, the frequency of refresh operation can be reduced, whichleads to an effect of suppressing power consumption.

Although there is no particular limitation on a substrate that can beused as the substrate 400 having an insulating surface, a glasssubstrate of barium borosilicate glass, aluminoborosilicate glass, orthe like is used.

In the bottom-gate transistor 410, an insulating layer serving as a basefilm may be provided between the substrate and the gate electrode layer.The base film has a function of preventing diffusion of an impurityelement from the substrate, and can be formed to have a single-layerstructure or a stacked-layer structure using one or more layers selectedfrom a silicon nitride layer, a silicon oxide layer, a silicon nitrideoxide layer, and a silicon oxynitride layer.

The gate electrode layer 401 can be formed to have a single-layerstructure or a stacked-layer structure using a metal material such asmolybdenum (Mo), titanium (Ti), chromium (Cr), tantalum (Ta), tungsten(W), aluminum (Al), copper (Cu), neodymium (Nd), or scandium (Sc), or analloy material including any of these as a main component.

The gate insulating layer 402 can be formed to have a single-layerstructure or a stacked-layer structure using any of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, an aluminum oxide layer, an aluminum nitride layer,an aluminum oxynitride layer, an aluminum nitride oxide layer, and ahafnium oxide layer by a plasma CVD method, a sputtering method, or thelike. For example, a silicon nitride layer (SiN_(y) (y>0)) with athickness of greater than or equal to 50 nm and less than or equal to200 nm is formed by a plasma CVD method as a first gate insulatinglayer, and a silicon oxide layer (SiO_(x) (x>0)) with a thickness ofgreater than or equal to 5 nm and less than or equal to 300 nm is formedas a second gate insulating layer over the first gate insulating layer,so that a gate insulating layer with a thickness of 200 nm in total isformed.

A conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b can be formed using an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, an alloy including any of these elementsas a component, an alloy film including a combination of any of theseelements, or the like. Alternatively, a structure may be employed inwhich a high-melting-point metal layer of Ti, Mo, W, or the like isstacked on one or both of a top surface and a bottom surface of a metallayer of Al, Cu, or the like. In addition, heat resistance can beimproved by using an Al material to which an element (such as Si, Nd, orSc) which prevents generation of a hillock or a whisker in an Al film isadded.

Alternatively, the conductive film to be the source electrode layer 405a and the drain electrode layer 405 b (including a wiring layer formedusing the same layer as the source electrode layer 405 a and the drainelectrode layer 405 b) may be formed using a conductive metal oxide. Asthe conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO), an indium oxide-zinc oxide alloy (In₂O₃—ZnO), orany of these metal oxide materials including silicon oxide can be used.

As the insulating layer 407, typically, an inorganic insulating filmsuch as a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, or an aluminum oxynitride film can be used.

As the protective insulating layer 409, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

In addition, a planarization insulating film may be formed over theprotective insulating layer 409 in order to reduce surface unevennessdue to the transistor. As the planarization insulating film, an organicmaterial such as polyimide, acrylic, or benzocyclobutene can be used.Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material) or the like. Notethat the planarization insulating film may be formed by stacking aplurality of insulating films formed using any of these materials. Notethat a needed component such as a reflective conductive layer or aliquid crystal layer may be provided as appropriate over the protectiveinsulating layer 409.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of the above embodimentswill be described.

FIG. 8A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 8Acan have a function of displaying various kinds of information (such asa still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, time, or the like on the display portion,a function of operating or editing the information displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Note that in FIG. 8A, a battery9635 and a DCDC converter 9636 (hereinafter abbreviated to a converter)are included in the charge and discharge control circuit 9634, as anexample.

When a semi-transmissive liquid crystal display device is used as thedisplay portion 9631, the electronic book reader is expected to be usedin a relatively bright environment, in which case the structureillustrated in FIG. 8A is preferable because power generation by thesolar cell 9633 and charge in the battery 9635 can be efficientlyperformed. Note that the solar cell 9633 can be configured so that thebattery 9635 is charged on a front surface and a rear surface of thehousing 9630, which is preferable. When a lithium ion battery is used asthe battery 9635, there is an advantage of downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 8A will be described with reference toa block diagram in FIG. 8B. The solar cell 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are illustrated in FIG. 8B, and the battery 9635, theconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar cell 9633 with the use of outside light is described. Thevoltage of power generated by the solar cell 9633 is raised or loweredby the converter 9636 so as to be voltage for charging the battery 9635.Then, when the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to be voltageneeded for the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Next, an example of operation in the case where power is not generatedby the solar cell 9633 with the use of outside light is described. Thevoltage of power accumulated in the battery 9635 is raised or lowered bythe converter 9637 by turning on the switch SW3. Then, power from thebattery 9635 is used for the operation of the display portion 9631.

Note that although the solar cell 9633 is described as an example of ameans for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar cell 9633 andanother means for charge may be used.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2010-009639 filed with Japan Patent Office on Jan. 20, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for driving a liquid crystaldisplay device comprising a plurality of pixels, wherein each of thepixels comprises a light-transmitting pixel portion and a reflectivepixel portion, wherein the light-transmitting pixel portion comprises afirst pixel transistor whose first terminal is electrically connected toa signal line and whose gate is electrically connected to a scan line,and a first liquid crystal element and a first capacitor which areelectrically connected to a second terminal of the first pixeltransistor, and wherein the reflective pixel portion comprises a secondpixel transistor whose first terminal is electrically connected to thesecond terminal of the first pixel transistor and whose gate iselectrically connected to a selection line, and a second liquid crystalelement and a second capacitor which are electrically connected to asecond terminal of the second pixel transistor, the method for drivingthe liquid crystal display device, comprising the steps of: in a firstperiod, turning on the first pixel transistor, turning off the secondpixel transistor, and supplying a first image signal to the first liquidcrystal element and the first capacitor from the signal line; in asecond period, performing display in the light-transmitting pixelportion in response to the first image signal; in a third period,turning on the first pixel transistor, turning on the second pixeltransistor, and supplying a signal for black display to the secondliquid crystal element and the second capacitor from the signal line inthe reflective pixel portion; performing display for a moving imagethrough the first to third periods; in a fourth period, turning on thefirst pixel transistor, turning on the second pixel transistor, andsupplying a second image signal to the first liquid crystal element, thefirst capacitor, the second liquid crystal element, and the secondcapacitor from the signal line; in a fifth period, performing display inthe reflective pixel portion in response to the second image signal; andperforming display for a still image through the fourth and fifthperiods.
 2. The method for driving a liquid crystal display device,according to claim 1, wherein the first image signal supplied in thefirst period is an image signal corresponding to any color of R, G, andB, and wherein a backlight which emits the corresponding color of R, G,and B is sequentially operated in the second period.
 3. The method fordriving a liquid crystal display device, according to claim 1, whereinthe signal for black display supplied in the third period is supplied toeach pixel by line sequential driving.
 4. The method for driving aliquid crystal display device, according to claim 1, wherein the secondimage signal is an image signal for displaying an image at a lowergrayscale level than an image of the first image signal.
 5. The methodfor driving a liquid crystal display device, according to claim 1,wherein time for displaying one image in the fourth and fifth periods islonger than time for displaying one image in the first to third periods.6. The method for driving a liquid crystal display device, according toclaim 1, wherein supply of a driver circuit control signal for drivingthe scan line and the signal line is stopped in the fifth period.
 7. Themethod for driving a liquid crystal display device, according to claim1, further comprising the step of: switching between a first modeincluding the first to third periods and a second mode including thefourth and fifth periods.
 8. The method for driving a liquid crystaldisplay device, according to claim 1, further comprising the step of:switching between a first mode including the first to third periods anda second mode including the fourth and fifth periods on basis of thefirst or second image signal.
 9. The method for driving a liquid crystaldisplay device, according to claim 1, wherein a frame frequency ishigher than or equal to 60 Hz in the first to third periods.
 10. Themethod for driving a liquid crystal display device, according to claim1, wherein a frame frequency is lower than or equal to 0.017 Hz in thefourth and fifth periods.
 11. The method for driving a liquid crystaldisplay device, according to claim 1, further comprising the step of:controlling a backlight by an optical sensor in the fifth period.
 12. Amethod for driving a liquid crystal display device comprising aplurality of pixels, wherein each of the pixels comprises alight-transmitting pixel portion and a reflective pixel portion, whereinthe light-transmitting pixel portion comprises a first pixel transistorwhose first terminal is electrically connected to a signal line andwhose gate is electrically connected to a scan line, and a first liquidcrystal element and a first capacitor which are electrically connectedto a second terminal of the first pixel transistor, and wherein thereflective pixel portion comprises a second pixel transistor whose firstterminal is electrically connected to the second terminal of the firstpixel transistor and whose gate is electrically connected to a selectionline, and a second liquid crystal element and a second capacitor whichare electrically connected to a second terminal of the second pixeltransistor, the method for driving the liquid crystal display device,comprising the steps of: in a first period, turning on the first pixeltransistor, turning off the second pixel transistor, and supplying afirst image signal to the first liquid crystal element and the firstcapacitor from the signal line; in a second period, turning off thefirst pixel transistor, and controlling a backlight in response to thefirst image signal; in a third period, turning on the first pixeltransistor, turning on the second pixel transistor, and supplying asignal for black display to the second liquid crystal element and thesecond capacitor from the signal line in the reflective pixel portion;performing display for a moving image through the first to thirdperiods; in a fourth period, turning on the first pixel transistor,turning on the second pixel transistor, and supplying a second imagesignal to the first liquid crystal element, the first capacitor, thesecond liquid crystal element, and the second capacitor from the signalline; in a fifth period, turning off the first pixel transistor, turningoff the second pixel transistor; and performing display for a stillimage through the fourth and fifth periods.
 13. The method for driving aliquid crystal display device, according to claim 12, wherein the firstimage signal supplied in the first period is an image signalcorresponding to any color of R, G, and B, and wherein a backlight whichemits the corresponding color of R, G, and B is sequentially operated inthe second period.
 14. The method for driving a liquid crystal displaydevice, according to claim 12, wherein the signal for black displaysupplied in the third period is supplied to each pixel by linesequential driving.
 15. The method for driving a liquid crystal displaydevice, according to claim 12, wherein the second image signal is animage signal for displaying an image at a lower grayscale level than animage of the first image signal.
 16. The method for driving a liquidcrystal display device, according to claim 12, wherein time fordisplaying one image in the fourth and fifth periods is longer than timefor displaying one image in the first to third periods.
 17. The methodfor driving a liquid crystal display device, according to claim 12,wherein supply of a driver circuit control signal for driving the scanline and the signal line is stopped in the fifth period.
 18. The methodfor driving a liquid crystal display device, according to claim 12,further comprising the step of: switching between a first mode includingthe first to third periods and a second mode including the fourth andfifth periods.
 19. The method for driving a liquid crystal displaydevice, according to claim 12, further comprising the step of: switchingbetween a first mode including the first to third periods and a secondmode including the fourth and fifth periods on basis of the first orsecond image signal.
 20. The method for driving a liquid crystal displaydevice, according to claim 12, wherein a frame frequency is higher thanor equal to 60 Hz in the first to third periods.
 21. The method fordriving a liquid crystal display device, according to claim 12, whereina frame frequency is lower than or equal to 0.017 Hz in the fourth andfifth periods.
 22. The method for driving a liquid crystal displaydevice, according to claim 12, further comprising the step of:controlling a backlight by an optical sensor in the fifth period.